In the manufacture of paper, an aqueous cellulosic suspension or slurry is formed into a paper sheet. The cellulosic slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1%, and often below 0.5% ahead of the paper machine, while the finished sheet must have less the 6 weight percent water. Hence the dewatering and retention aspects of paper making are extremely important to the efficiency and cost of the manufacture.
More specifically, the slurry is an aqueous suspension containing cellulosic material and in some cases selected mineral pigments. This slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1%, and often below 0.5% ahead of the paper machine. Associated with papermaking slurries, called furnishes, is a large variation in the size and shape of the particles present. These particles may range in size from less than one micrometer for many mineral pigments or fillers, up to several millimeters in their largest dimension for fibers. The initial dewatering of a paper furnish typically takes place by the ejection of the cellulosic furnish onto or between filter fabric(s), called the wire. The openings in these wires are typically on the order of 200 mesh, which corresponds to a hole size capable of passing particles with a diameter of 76 micrometers. If no forces of attraction exist between particles, the mineral pigments would very easily pass through the wire and would not be retained in the sheet, compromising the benefits for which the mineral pigments were added. Thus, under normal papermaking circumstances, many components of the furnish that are small enough to pass through the openings in the wire will require modification if they are to remain in the sheet.
As the fibers form a mat on the wire, they generate their own filter medium and many of the smaller particles in the furnish may be trapped by simple filtration in the fiber mat, particularly if the sheet is thick, i.e. high basis weight. However, even if the basis weight is high, a significant fraction of the small particulate material may not be adequately retained. When basis weights are low or machine turbulence prevents mat formation, the filtration mechanism of small particle retention is extremely inadequate. Under papermaking circumstances when the filtration mechanism is inadequate, chemical treatments generally called retention aids are required to modify the interparticle interactions thereby resulting in coagulation and/or flocculation of the particles.
Retention of small particulate components leads to numerous benefits for the papermaker. Mineral fillers like clay and calcium carbonate are often less expensive than fibers, and substitution of such fillers for fiber provides a way for the papermaker to reduce the raw material costs. Retention of fillers and fiber fines is also necessary to achieve the sheet properties needed for a given end use. Such properties might include sheet opacity, brightness, and appropriate ink interactions. Because the small particles have large surface areas for a given mass, significant amounts of additives such as dyes or sizing agents can be attached to them making retention of the fines necessary for effective utilization of such additives.
Filler particles and fiber fines which are not retained initially, or in the so called first pass, are to a large extent recycled via the white water system back into the furnish, increasing the fraction of small particles present in the furnish over time. This result is often unsatisfactory for several reasons. Some important and expensive materials lose their effectiveness upon recycling in the white water system, and their retention in the first pass is needed for performance or sheet properties. Examples of such materials are titanium dioxide and alkaline sizing agents. Although the total amount of fines in the sheet may be increased in this way, their distribution in the sheet will tend to be very uneven frequently resulting in two-sided phenomena of the paper. In addition, the concentration of unretained materials in a papermachine's white water system can contribute to deposit problems and related runnability problems which result in lost or slowed production and poor product quality. These problems are remedied by using effective retention aids, resulting in a machine with improved runnability, more efficient use of fiber and filler raw materials, and less waste to the mill's waste treatment facility.
Greater retention of fines, fillers, and other slurry components permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of lower quality are employed to reduce paper making costs, the retention aspect of paper making becomes even more important because the fines content of such lower quality pulps is greater generally than that of pulps of higher quality. Greater retention also decreases the amount of such substances lost to the white water and hence reduces the amount of material wastes, the cost of waste disposal and the adverse environmental effects therefrom. It is desirable to reduce the amount of material employed in a paper making process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
Another phenomena of primary interest in papermaking is dewatering. The dewatering method of the least cost in the process is gravity drainage, and thereafter more expensive methods are used, for instance vacuum, pressing, felt blanket blotting and pressing, evaporation and the like, and in practice a combination of such methods are employed to dewater, or dry, the sheet to the desired water content. Since gravity drainage is both the first dewatering method employed and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Dewatering generally, and particularly dewatering by drainage, is believed to be improved when the pores of the paper web are less plugged, and it is believed that retention of small particles by adsorption to the fibers in comparison to retention by filtration reduces such pore plugging.
Another important characteristic of a given paper making process is the formation of the paper sheet produced. Formation is determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90%, the formation parameter generally abruptly declines from good formation to poor formation.
In order to improve retention and drainage in papermaking a flocculant is introduced to induce flocculation. Flocculation describes a number of possible strategies which result in agglomeration of these previously mentioned mall particles. Different degrees of flocculation is required at each stage of operation in pulp and paper mills. At the forming wire on the paper machine, paper is formed by the rapid dewatering of the paper making slurry. This slurry is generally comprised of fibers, fines, mineral fillers and other additives. Under normal conditions, more than 50% of components of the slurry are small enough to pass through the forming wire. In order to retain the smaller components within the structure of the sheet having a low degree of two-sidedness, polymeric retention aids are being used. Such retention aids operate by flocculating of the components of the slurry before the slurry is consolidated as the sheet in the consecutive dewatering stages. The proper level of flocculation is necessary to provide the required retention and drainage rate while not significantly degrading the sheet uniformity-formation.
Various characteristics of the slurry, such as pH, hardness, ionic strength, cationic demand, may affect the performance of a flocculant in a given application. The choice of flocculant involves consideration of the type of charge, charge density, molecular weight, type of monomers and is particularly dependent upon the water chemistry of the mill system being treated.
Hydrolyzable aluminum salts are used extensively as coagulants in papermaking. Because of the acid generated by the aluminum hydrolysis, the pH of machines using alum is depressed, and the process is referred to as "acid papermaking". The aluminum species possessing the greatest coagulating ability are formed in the pH range of 4 to 6. Polyaluminum chlorides are also effective coagulants. Being partially neutralized, they do not depress the pH to the extent that alum does and are generally more applicable over a wider pH range.
In a single polymer program, a flocculant, typically a cationic polymer, is the only material added. Another method of improving the flocculation of cellulosic fines, mineral fillers and other furnish components on the fiber mat is the dual polymer program, also referred to as a coagulant/flocculant system, added ahead of the paper machine. In such a system there is first added a coagulant, for instance a low molecular weight synthetic cationic polymer or cationic starch to the furnish, followed by the addition of a flocculant. Such flocculants generally are a high molecular weight synthetic polymers which bind the particles into larger agglomerates. The presence of such large agglomerates in the furnish as the fiber mat of the paper sheet is being formed increases retention. The agglomerates are filtered out of the water onto the fiber web, whereas unagglomerated particles would to a great extent pass through such paper web.
In systems containing high concentrations of anionic polymeric/oligomeric substances, the performance of cationic polymers is often detrimentally affected. These anionic substances may be of inorganic or organic origin. Silicates used as hydrogen peroxide stabilizers in pulping, bleaching, and de-inking processes and species extracted from the wood like polygalacturonic acids and lignin derivatives are the most typical examples of components of anionic detrimental substances, also called "anionic trash". Nonionic and anionic polymers are affected by these substances to a much lower degree than cationic polymers.
An example of a papermaking program which utilizes a nonionic flocculent is disclosed by Linhart et al., U.S. Pat. No. 4,772,359 as a process to increase drainage rate and the retention of fillers, fines and pigments which comprises adding to the pulp slurry an effective amount of a high molecular weight water-soluble polymer of an N-substituted vinylamide. It is well known that vinylamides, in the presence of acid, can hydrolyze to yield a substance which contains cationic moiety. Cationic moieties are very effective at inducing flocculation in papermaking slurries as well as inducing flocculation in these system.
The Linhart et al. reference does not show that a combination of resin and nonionic homopolymer acrylamide may be utilized advantageously. Poly(acrylamide) is only used as a control in these examples.
Upon reference to Table 4 of the '359 patent, it is apparent that cationic polyacrylamide and resin in combination do not provide any added performance over polymer alone. For polymer I, drainage time decreases by 1 unit, from 89 to 88. Optical transparency increases from 53 to 57, a change of 4 units. Both of these changes are within experimental error, and thus do not illustrate any advantage of adding polymer and resin together. One skilled in the art reading this reference and analyzing this set of data would not pursue such a combination, based on the lack of increased efficiency demonstrated by Table 4.
Upon reference to Table 5, it is apparent that the use of nonionic polyacrylamide does not lead to any increase in efficiency. If the polymer and resin combination is compared to phenol alone, drainage is decreased from 139 to 138, a change of only one unit. The optical transparency decreased from 35 to 31, a change of four units. Both of these results are within experimental error, and actually teach that the addition of resin and polymer do not provide any advantages over the addition solely of resin. Furthermore, the optical transparency data would suggest that the resin/polyacrylamide combination negatively impacts retention as evidenced by a decrease in optical transparency. However, this interpretation also does not consider the inherent error associated with the experimental method. Therefore, one skilled in the art analyzing Table 5, would not be taught that non-ionic polymer/resin combinations increase efficiency.
Therefore an examination of the data of Tables 4 or 5 of the Linhart et al. reference would not lead one skilled in the art to believe that there would be any inherent advantage to a combination of polymer and resin, when the polymer is a cationic or non-ionic polyacrylamide, for this reference illustrates no effect. One skilled in the art upon reading the Linhart reference would therefore not pursue the use of a combination when attempting to ameliorate the operation of the papermaking systems described by the instant invention.
Another example of a dual polymer system utilizing a nonionic flocculant is the polyethylene oxide (PEO) and cofactor program. PEO is an effective retention aid for newsprint and other mechanical pulp furnishes. Known cofactors include kraft lignin, sulfonated kraft lignin, naphthalene sulfonate, tannin extract, and water-soluble phenol-formaldehyde resins. A recent EPO patent application (Echt, EP 621 369 A1, 1995), discloses using poly(p-vinyl phenol) as a cofactor.
The method disclosed in the Carrard et al., U.S. Pat. No. 4,070,236 describes the use of poly(ethylene oxide), referred to as PEO, having a molecular weight in excess of 1,000,000 with water soluble phenol-formaldehyde or naphthol-formaldehyde resins or sulphur resins. The Carrard et al reference also discusses the use of other polymers in conjunction with the above mentioned two-component program. Such polymers include polyamide amine, polyalkylene imine, polyamine (all cationic) and polyacrylic-polyacrylamide copolymer (anionic).
In the APPITA Annual General Conference report, 83-90, 1995, an improvement in the performance of PEO/phenolic enhancer programs was discussed. The improvement was the result of adding cationic polyacrylamide to PEO/phenolic enhancer programs. The synergy exists between the PEO/resin combination and the cationic polyacrylamide.
However, there are problems associated with the use of PEO as a retention aid. PEO is expensive when compared to many synthetic flocculants. Also, PEO chains are susceptible to degradation which results in lowering the molecular weight and thus flocculation efficiency. Degradation can be caused by either shear forces or extended storage. In addition, PEO is susceptible to oxidizing agents that may be present in the furnish.
In an attempt to circumvent these difficulties, Huinig Xiao and R. Pelton, reported synthesis of a nonionic copolymer of acrylamide and poly(ethylene-glycol) methacrylate. This copolymer contains pendant PEG chains intended to impart PEO like character and thus activity, as claimed by Xiao and Pelton, via interaction with resole-type phenolic enhancer to form the three dimensional structures responsible for its good performance as a retention polymer. However, Xiao and Pelton did not report any beneficial effect from the use of phenolic enhancer on flocculation performance of polyacrylamide homopolymers. This information has been presented in PCT/CA94/00021.
Furthermore, flocculation can be beneficial in applications other than wet-end papermaking. Among these are applications such as saveall clarification. The save-all is used to separate solids which are agglomerated in the white water and keep such solids within the paper making system. Proper operation of the save-all is very important for economical use of cellulosic raw materials, fines and other additives. It is also important to minimize the environmental impact of the effluent stream with lower suspended solids, lower COD and BOD values and reduced amounts of solid waste materials.
Clarifiers, dissolved air floatation units (DAF), are used to separate the suspended and colloidal solids from the waste water streams from paper mills, pulp mills, and de-inking facilities. Effective solids removal allows for an increase in the recycling of water used in the system, thereby reducing the consumption of fresh water.
Flocculation is also used in sludge dewatering presses. The presses are used to concentrate the solid waste materials. The appropriate operation of such presses reduces the costs and other problems associated with the disposal of solid waste materials and lowers the environmental impact of such materials.
The most significant flocculation applications include alum and derivatives of aluminum, single cationic polymer programs, dual polymer programs, and microparticle programs.
The present invention departs from previously disclosed claims regarding papermaking as well as other applications where separation of solids from aqueous liquids is important. This patent discloses the novel use of a phenolic enhancer to be added to a papermaking slurry either before or after a period of high shear. The phenolic enhancer can also be added either before or after a flocculant. The flocculants used may be either anionic or nonionic. A synergistic interaction is observed when the phenolic enhancer and the flocculant are added in the disclosed manner. This unique combination of components as well as their mode of addition constitute the novel, surprising and unexpected invention not obvious to one skilled in the art disclosed herein. This invention allows improved levels of retention, formation, uniform porosity, and overall dewatering in the papermaking process. Furthermore, this process is responsible for improved flocculation. While the invention has been, and will be described particularly in reference to the manufacture of paper those skilled in the art will readily appreciate that the method using the phenolic enhancer and water soluble polymeric flocculant will be applicable to a wide variety of processes in which solids are separated from aqueous liquids, or conversely when aqueous liquids are separated from solids. The improved separation techniques taught herein can be beneficially applied to applications other than pulp and paper systems, for example, where ever solid/liquid separation or emulsion breaking are performed. Examples of such applications are municipal and industrial sludge dewatering, clarification or raw waters, the dewatering of aqueous mineral slurries, the removal of oils and greases from waste waters and the like.